Cellular solid polymers, often referred to as "foams" can be prepared by generating a gas during the polymerization of the liquid reaction mixture. The gas generated causes foaming of the reaction material which is normally in a plastic or liquid state. The polymerization reaction continues while the foaming occurs until the polymer sets or gels into the cellular pattern formed by the foam bubbles. The solidified polymer thus becomes a cellular solid mass popularly known as a "foam". Polyurethane foams are generally prepared by the reaction of an active hydrogen-containing compound and a polyisocyanate, in the presence of a blowing agent such as water, and usually, a reaction catalyst and foam stabilizer.
Polyurethane foam is used widely as a flexible cellular product in the comfort cushion market (furniture, bedding, automotive); in the textile area (apparel, blankets); in the industrial packaging and insulating fields; in other household furnishings and sponges; filters, and the like. The versatility of polyurethane foam, permitting its use in diverse markets, results in substantial part from the nature and variety of the raw materials which are used to produce the foam products, as well as the manner in which the raw materials and the resultant foam are processed. Foams ranging widely in density and hardness, in tensile and tear properties, in resistance to compression set and fatigue, in resilience and hysteresis, in durability and toughness are obtained by selection and variation in raw materials and processing conditions. An important further characteristic in foam that likewise varies widely is its breathability, or resistance to air flow, of the basic cellular structure.
The cellular solid polymer foam has a skeletal framework of relatively heavy strands forming an outline for the cell structure. The strands of the skeletal framework are conventionally connected by very thin membranes, or windows, which form the walls of the cells. In open-celled foams, some of the windows are open or torn in each cell thus forming an interconnecting network open to air flow. However, conventional polyurethane foams are not sufficiently porous or open-celled to exhibit very low resistance to air flow therethrough which are required for many utilities, such as filtering. Accordingly, in attempting to improve the properties of such open-celled foams in a desired direction, the art has tried various post-forming methods of reticulating, or increasing the degree of openness, by breaking or removing the residual cell windows of such foams. Chemical, mechanical and thermal reticulation means have all been used.
For example, removing cell walls has been suggested by using the hydrolyzing action of water in the presence of an alkali metal hydroxide. By carefully adjusting the conditions during the hydrolysis reaction, it has been demonstrated that cell windows can be removed without adversely affecting the skeletal framework. Reticulation can also be carried out by melting the windows by, for example, a high temperature flame front to heat the cell windows or walls to above the melting point of the polymer. Thus, it was proposed that by carefully regulating the conditions under which this process is carried out, the cell windows can be melted without adversely affecting or melting the skeletal strands.
Various purely mechanical means to reticulate both flexible and rigid foams have also been suggested. For example, the art has utilized a procedure of compressing, mangling or wringing a flexible foam to open the pores to render it more useful as a sound insulating or sound absorbing medium. Foams have been made more open, to improve sound absorbing properties, by heating with super-heated steam at 140.degree. C., or by blowing with compressed air or high velocity liquids.
Whatever post treatment is used, it must of necessity produce some effect upon the stalk or skeletal structure of the foam regardless of how minimal such effect is. In many cases, when working with a pigmented foam, the color intensity and hue are substantially changed by the post-polymerization reticulation treatment. This can result in some difficulty in color matching for certain applications. Moreover, post-treatment methods add significantly to the cost of the foam. Therefore, a method for producing a substantially open-celled nonlustrous foam in situ during the reaction or foaming process, without the necessity of an additional post polymerization treatment step, has long been sought and would be an advance in the art.
The art has been at least partially successful in obtaining open-celled polyether type polyurethane wherein the polyurethane is prepared using a polyhydric polyether as the active hydrogen-containing reactant. For example, in U.S. Pat. No. 3,433,752, an open-celled, rigid, polyether polyurethane foam is produced by the addition of an alkali metal salt of a sulfonated high molecular weight fatty acid. In Canadian Pat. No. 797,893, the preparation of a polyalkylene ether polyurethane having an open structure is disclosed which includes the addition of a petroleum hydrocarbon liquid, e.g., kerosene or mineral oil, as a cell opening agent which causes the cell membranes of the foam to rupture during the foaming process, thus allegedly providing an open material. The intent of this process is to prevent shrinking of the foam during cooling which often occurs with a substantially fully closed cell structure. However, the above process is explicitly limited to polyalkylene ether polyurethane and does not result in a completely open structure, but merely one in which sufficient membranes are removed to permit at least some air permeability throughout the internal foam structure. Also see U.S. Pat. No. 3,454,504.
In Canadian Pat. No. 797,892, an open-celled polyether polyurethane foam material is obtained by the reaction of a polyhydric polyether compound with an organic polyisocyanate and blowing agent in the presence of an inert organic liquid solvent such as methylene chloride, acetone, hexane or pentane.
U.S. Pat. No. 3,178,300 describes a process for preparing "skeletal" polyurethane foam by mixing an organic polyisocyanate with castor oil in the presence of an alkyl silane oxyalkylene block copolymer (a surfactant), a blowing agent and a monohydric organic compound, such as a monohydric alcohol or monocarboxylic acid. This material has a limited usefulness, however, because of the low structural strength caused by the chain-stopping monohydric additive, which limits its strength, and the relatively coarse cell structure. Further, castor oil is notoriously difficult to use as a sole polyhydroxy reactant. The reaction with castor oil is highly exothermic, often causing scorching or even burning of the foam, and often the formation of odoriferous by-products.
U.S. Pat. No. 3,165,483 describes a process for making a skeletal foam by reacting a polyisocyanate with either castor oil or a polyhydric polyether in the presence of a silane-oxyalkylene block copolymer and of an unreactive hydrocarbon or halohydrocarbon, ester, aldehyde and/or ketone. These materials are also indicated to be useful as filters.
U.S. Pat. No. 3,748,288 describes production of an open polyester-type urethane foam by the addition of a minor proportion of a polyether polyol reagent and a small amount of a hydrophobic, anti-foaming organo-silicone compound. U.S. Pat. No. 3,884,848 describes the replacement of the hydrophobic, anti-foaming organo-silicone compound with at least one ester of the formula (RCOO).sub.n R', wherein R and R' are alkyl or alkenyl groups having from one (1) thirty (30) carbon atoms, at least three carbon atoms, and n is an integer from one (1) to three (3).